1. Field of the Invention
The present invention relates to cryogenic cooling systems. More specifically, the present invention relates to systems and techniques for reducing vibration generated by cryogenic coolers.
While the present invention is described herein with reference to illustrative embodiments for particular applications, it should be understood that the invention is not limited thereto. Those having ordinary skill in the art and access to the teachings provided herein will recognize additional modifications, applications, and embodiments within the scope thereof and additional fields in which the present invention would be of significant utility.
2. Description of the Related Art
Cryogenic coolers are used to cool infrared detectors to cryogenic temperatures between 40 to 80 degrees Kelvin. For this purpose, a Stirling cycle cooler is often used. A conventional Stirling cycle engine consists of a compressor piston with a cylinder, an expansion piston with a cylinder, and a drive mechanism. The drive mechanism converts rotary motion of a motor and crankshaft to a reciprocating motion of the two pistons ninety degrees out-of-phase. A regenerator and a crankcase housing are also included. Cooling is effected by the expansion cycle of a gas at the regenerator/expander assembly.
The basic Stirling cycle engine technology is employed in a Split-Stirling cooler with the exception that the reciprocating displacer piston and cylinder located within the expander are physically separated from the compressor and the regenerator is located within the displacer piston. The reciprocating displacer piston within the expander and the compressor are then interconnected with a small diameter gas transfer line which is sufficiently flexible to avoid the introduction of excessive spring torque to the system. This design permits the compressor, which is large compared to the expander, to be located remotely where available volume and heat rejection capability exists. The Split-Stirling cryogenic cooler is electrically driven so that gas pressure differentials on opposite sides of the displacer piston and cylinder provide the motive force to the cryogenic cooler.
Unfortunately, although the Stirling cycle cooler offers adequate lifetime cooling capacity in an efficient mechanical operation, the operation of the cooler often induces vibration in the detector being cooled.
Accordingly, there has been a need in the art for a system or technique for minimizing vibration from Stirling cycle coolers.
This need was addressed somewhat by a Split-Stirling cycle cooler in which both the compressor and expander modules are designed to achieve a low vibration level by incorporating an opposed reaction mass/actuator within the same housing to obtain a near perfect balance in all active forces. The imbalance forces are then controlled by a simple position matching servo-control system.
Unfortunately, because of nonlinearities in the motor drive electronics, the piston suspension flexures, and gas thermodynamics, the resulting vibrations contain high-order harmonics in addition to the fundamental drive frequency. At these high-order harmonic frequencies, the dynamic balance condition, referred to above, does not hold and the harmonic vibrations disturb the operation of the detector being cooled. Accordingly, a blur of the image output by the cooled detector results.
A position loop servo is a servo-control system that controls the position (or amplitude) of a piston motion. While position loop servos may be used to attenuate the high-order harmonic vibrations to some extent, position loop servos typically fail to provide adequate attenuation because of a limited position loop servo-bandwidth.
Another proposed solution to the problem of high-order harmonic vibration involves an increase in the position loop servo bandwidth. An increase in position loop servo-bandwidth can be achieved by increasing the gain in the forward path. While this approach offers some promise, motor drive dynamics and internal structural resonances of the suspension system prevent the implementation of higher bandwidth servos.
Electronic image motion correction for compensating these induced vibrations tends to be slow and expensive.
Thus, a need remains in the art for an effective system and/or technique for reducing vibration from a Stirling cycle cooler due to high-order harmonics of a fundamental frequency.